Preventing medical events is a critical public health concern, and adherence to recommended health behaviors can play a major role in such prevention. A key example of the importance of health behavior is in the secondary prevention of recurrent cardiovascular events in patients suffering an acute coronary syndrome (ACS). Despite the undeniable importance of health behaviors (e.g., physical activity, following a low-fat diet) in post-ACS patients, many are unable to follow these medical recommendations. Positive psychological states, especially optimism, have been linked to greater participation in healthy behaviors and superior cardiovascular outcomes in persons with and without heart disease. An emerging field of psychological interventions has focused on increasing the frequency and intensity of positive states, and these 'positive psychology' programs have consistently led to improvements in happiness, reductions in depression, and improvements in overall well-being in healthy individuals. However, such interventions have not been well-studied in ACS patients, and their effect on health behaviors has not been examined. Consistent with PA-11-063 (Translating Basic Behavioral and Social Science Discoveries into Interventions to Improve Health-Related Behaviors), we propose a multi-step treatment development project to utilize positive psychological interventions to improve specific health behaviors (physical activity, diet, and medication adherence) in patients initially hospitalized with ACS. We will perform qualitative research via semi-structured interviews to better understand the relationship between positive emotions and health behaviors in this cohort, and will design a positive psychological intervention that is suited to this population. We will then pilot test components of this intervention to assess initial feasibility/acceptability and understand the optimal components and 'dosing' of the intervention. Finally, we will end with a small randomized efficacy trial of our revised treatment in ACS patients. The trials in this grant will enroll a minimum of 208 subjects admitted with ACS, and will utilize a phone-based intervention to improve feasibility and reduce cost. The primary outcome measure for studies in this grant will be physical activity, as measured via an accelerometer. We selected this outcome because of the importance of this health behavior and the substantial difficulty that many ACS patients have in initiating and maintaining a physical activity regimen, and because positive emotions have been linked to greater physical activity. Given that the intervention may have broader effects, however, we will also measure the impact of our intervention on following a heart-healthy diet (evaluated via structured assessment) and adhering to medication (aspirin, as measured by cap openings). By the end of the grant period, we will have (in concordance with the PA) created a viable positive psychology- based intervention that will be ready for testing in a larger, multisite efficacy trial that is adequately powered to detect significant improvements in physical activity and other important health behaviors in ACS patients.